The present invention relates to a method for generating X-rays in an X-ray tube used for X-ray computed Tomographs and the like, and to an X-ray tube for performing such method.
In order to reduce the time of diagnosis by means of an X-ray computed Tomograph and to improve a resolving power of an image obtained by an X-ray computed tomograph, it has been desired to have an X-ray target of a rotating anode in an X-ray tube large enough to increase the heat capacity of the X-ray tube. For example, a X-ray target operable at an average temperature of about 1,200.degree. C. has been desired.
In conventional X-ray tubes, there was a problem that, if the X-ray target was large enough to increase the heat capacity thereof, the X-ray target was considerably deformed by heat toward a side opposite to a side of the X-ray target against which electron beams were applied, with the result that an effective amount of X-rays obtained by the X-ray tube was not increased for the increase in heat capacity. Further, if the heat capacity of the X-ray tube is increased, the cooled capacity of the X-ray target must be also increased; to this end, the X-ray target must be rotated at a high speed of the order of about 10,000 r.p.m. In the conventional X-ray target, since the X-ray target has a heavy weight, if it is rotated at high speed, bearings for supporting the X-ray target are subjected to considerable load, which causes the wear of the bearings, resulting in eccentric rotation of the X-ray target. This inconvenience also creates the problem that the effective amount of X-rays obtained by the X-ray tube is not increased for its heat capacity being increased. Further, in this case, there also arises a problem that the withstand voltage of the X-ray tube is reduced due to metal powder from worn bearings.
An X-ray target having a base made of graphite to reduce the weight of the target is already known. However, such X-ray target has been used to be rotated at a speed less than approximately 5,000 r.p.m.; thus, in this conventional light-weight X-ray target, since no consideration has been given to the high speed rotation thereof, if such an X-ray target is rotated at a high speed of approximately 10,000 r.p.m., cracks and the like will be generated; thus, such an X-ray target cannot be used in safety.